This invention relates to the sensing of the level of contact between parts of the human body and a sensor. In the prior art, the types of technologies that sense the level of human body contact have incorporated technology that uses various semiconductor devices, such as those shown in U.S. Pat. No. 4,353,056 (Tsikos), entitled “Capacitive Fingerprint Sensor,” and U.S. Pat. No. 5,325,442 (Knapp), entitled “Fingerprint Sensing Device And Recognition System Having Predetermined Electrode Activation.”
One problem with these types of sensors is that the semiconductor devices used for the sensors have the liability of being sensitive to contamination and being vulnerable to damage by external forces. As a result, technologies that are independent of semiconductor sensors are considered more desirable.
Such a prior art sensor is shown in FIG. 1. Prior art technologies that sense the level of human body contact generally comprise a sensing component or components, shown as reference number 10 in FIG. 1, and a voltage component that provides operating voltage to the sensing component. Typically, the sensing component senses the level of human body contact and sends the outcome to an arbitrary electronic device, such as a signal processing chip (not shown).
The sensing component of the prior art sensor 10 of FIG. 1 comprises a sensing point 3, parasitic capacitor 1 and output device 2, for example, an inverter.
In operation, the user initiates contact with the device by placing a finger on sensing point 3, and becomes part of the circuit, in general appearing as a resistor 4 connected to ground. Any electrical charge stored in the parasitic capacitor 1 would flow through the human body resistive path 4 to ground, thereby discharging capacitor 1. As a result, the voltage level at node P1 would fall, causing the output of inverter 2 to rise, leading to a high output of sensor 10.
One problem with the prior art device of FIG. 1 is that the sensing device 10 is generally configured using a continuous activation method, in which capacitance 1 must re-charge while the human body is in contact with sensing point 3. After the charging of the capacitor 1 is completed, the determination of whether the human body is in contact with sensing point 3 is made by sensing whether the charging voltage is maintained.
When this method is used, the body of the user is in contact with sensing point 3 while capacitor 1 is being charged, and thus the user is subject to the danger of excess direct current flowing through the body. These kinds of direct currents, while generally not life threatening, could cause uneasiness and discomfort to the user.
From the viewpoint of device performance, the prior art configuration also causes some practical difficulties. If the amount of current flowing through the body is too great, the amount of power consumption will be much more than necessary, thereby increasing the total power consumption of the device. This is especially troublesome for portable type devices which may draw operational power from a battery.
Yet another problem with the prior art device exists in connection with the physical size and resolution of the device. Typically, sensing device 10 is connected to a signal processing chip, where there is a one-to-one correspondence between pins on the chip and a plurality of sensing devices 10. For the sensor to work with sufficient precision, the resolution should be at least 500 dpi. With a typical human finger being about 1 cm in width, there must be at least 400 sensing devices 10 to enable normal operation, and the signal processing chip must therefore have an equivalent number of individual pins for it to process the information from the sensing devices. This leads to excessively large devices and/or high production costs to incorporate the large number of connections in a reasonable area.
Because of these problems, the practical uses of the prior art devices are limited. It would therefore be advantageous and desirable to have a sensing device which alleviates the aforementioned problems extant in the prior art devices.